During drilling operations (e.g. for petroleum production), the pressure head of drilling fluid present in a borehole and up to a platform, may cause the liquid pressure in the lower portion of the borehole to become too high.
Excessive drilling fluid pressures may result in the drilling fluid causing undesirable damage to the formation being drilled (e.g. through drilling fluid penetrating into the formation).
The formation may also include special geological formations (saline deposits etc.) that require the use of special drilling fluid in order to stabilise the formation.
According to prior art, it is difficult to reduce the specific gravity of the drilling fluid in order to reduce the pressure to an acceptable level. In many cases, it has proven difficult to achieve a sufficient reduction in the specific gravity of the drilling fluid without causing an unacceptable degree of change in the physical properties of the drilling fluid, such as viscosity.
It is known to dilute the drilling fluid in a riser in order to reduce the drilling fluid pressure (see U.S. Pat. No. 6,536,540).